1. Field of the Invention
This invention relates to a failure determination system, a failure determination method, and an engine control unit for a variable-cylinder internal combustion engine which is capable of switching a cylinder operation mode between an all-cylinder operation mode in which all of a plurality of cylinders are operated and a partial-cylinder operation mode in which part of the plurality of cylinders are made idle, the failure determination system being capable of determining whether or not the valve-actuating system associated with the part of the plurality of cylinders for being made idle has failed.
2. Description of the Prior Art
A failure determination system of this kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 7-63097. In the variable-cylinder internal combustion engine disclosed in this publication, all the four cylinders of the engine are operated during the all-cylinder operation mode, whereas two of the four cylinders are made idle, i.e. inhibited from operating, during the partial-cylinder operation mode. In the exhaust system of the variable-cylinder engine, an oxygen concentration sensor is arranged for detecting the concentration of oxygen in exhaust gases. The oxygen concentration sensor delivers a signal indicative of the sensed oxygen concentration, based on which is calculated an actual air-fuel ratio. In the failure determination system, during the all-cylinder operation mode, the count of a failure counter is incremented if the ratio of the actual air-fuel ratio to a reference air-fuel ratio becomes equal to or smaller than a predetermined reference value, that is, if the actual air-fuel ratio is considerably richer than the reference air-fuel ratio. Then, if the count of the failure counter becomes equal to or larger than a predetermined value, it is determined that the valve-actuating system associated with the cylinders for being made idle has failed. This method of failure determination is based on a phenomenon that when the intake valves and exhaust valves of the cylinders for being made idle are in a normally closed state due to the failure of the valve-actuating system associated therewith, the air-fuel ratio of a mixture supplied to the other two cylinders is controlled to a richer value than when the valve-actuating system is normally operating, so that the actual air-fuel ratio undergoes a shift to the richer side.
In the above conventional failure determination system, however, all the cylinders are allowed to operate normally in the all-cylinder operation mode. In this state, it is determined that the valve-actuating system associated with the cylinders for being made idle has failed if the actual air-fuel ratio is shifted to a richer value indicative of failure of the valve-actuating system, since the air-fuel ratio of a mixture supplied to the other cylinders is controlled to a richer value due to the failure of the valve-actuating system of the cylinders for being made idle. Therefore, it takes much time to determine that the valve-actuating system has failed, after the failure has actually occurred. Further, under conditions in which the air-fuel ratio tends to be lean, e.g. when the engine temperature is low or fuel is difficult to vaporize depending on the type thereof, there can be a case where the actual air-fuel ratio is drifted to a leaner value to fall short of the above-mentioned richer value indicative of failure of the valve-actuating system, even though the valve-actuating system has actually failed. In such a case, there occurs an erroneous determination that the valve-actuating system is normally operating although it has failed.
It is an object of the invention to provide a failure determination system for a variable-cylinder internal combustion engine, which is capable of promptly and accurately carrying out failure determination of the valve-actuating system of cylinders for being made idle during the partial-cylinder operation mode.
To attain the above object, the invention provides a failure determination system for a variable-cylinder internal combustion engine including a plurality of cylinders each having at least one intake valve and at least one exhaust valve, and a valve-actuating system including a valve-actuating mechanism for actuating the at least one intake valve and the at least one exhaust valve of each of the cylinders, the engine being capable of switching a cylinder operation mode between an all-cylinder operation mode in which all of the plurality of cylinders are operated, and a partial-cylinder operation mode in which fuel supply to part of the plurality of cylinders is stopped and at the same time the part of the plurality of cylinders are made idle by stopping the valve-actuating mechanism from actuating the at least one intake valve and the at least one exhaust valve of each of the part of the cylinders.
The failure determination system is characterized by comprising:
oxygen concentration-detecting means for detecting a concentration of oxygen in exhaust gases exhausted from the plurality of cylinders;
failure-determining operation mode-executing means for executing a failure-determining operation mode for causing the valve-actuating mechanism to resume actuation of the at least one intake valve and the at least one exhaust valve of at least one of the part of the cylinders in a state in which the fuel supply to the part of the cylinders is stopped, when the cylinder operation mode is switched from the partial-cylinder operation mode to the all-cylinder operation mode; and
failure determination means for determining whether or not the valve-actuating system including the valve-actuating mechanism has failed, based on an oxygen concentration detected by the oxygen concentration-detecting means when the failure-determining operation mode is executed.
In this variable-cylinder internal combustion engine incorporating the failure determination system according to the first aspect of the invention, the cylinder operation mode is switched between the all-cylinder operation mode and the partial-cylinder operation mode. In the partial-cylinder operation mode, fuel supply to part of the plurality of cylinders by a fuel supply mechanism is stopped, and at the same time the valve-actuating mechanism associated with the part of the cylinders is stopped from actuating the intake valves and exhaust valves of the part of the cylinders, to thereby make the part of the cylinders idle in operation. According to the failure determination system, the failure-determining operation mode is executed by the failure-determining operation mode-executing means when the cylinder operation mode is switched from the partial-cylinder operation mode to the all-cylinder operation mode. In the failure-determining operation mode, the actuation of the intake valves and exhaust valves of at least one of the part of the cylinders is resumed in a state in which fuel supply to the part of the cylinders is stopped. Therefore, when the valve-actuating system associated with the part of the cylinders is normally operating, air taken into combustion chambers from the intake system via the intake valves is directly exhausted to the exhaust system of the engine via the exhaust valves, and hence an oxygen concentration detected at this time by the oxygen concentration-detecting means becomes higher than that detected before the start of the failure-determining operation mode.
On the other hand, when the valve-actuating system of the part of the cylinders has failed, the intake valves and exhaust valves thereof remain held in a closed state to inhibit air from being exhausted to the exhaust system, whereby the concentration of oxygen in exhaust gases is hardly changed in comparison with that before execution of the failure-determining operation mode. As described above, by execution of the failure-determining operation mode, if the valve-actuating system is normally operating, a state is forcibly produced in which the concentration of oxygen in exhaust gases should be made by far higher than before the execution of the failure determination operation mode, and based on the oxygen concentration detected at this time, determination as to whether or not the valve-actuating system has failed is carried out. This makes it possible to carry out the failure determination of the valve-actuating system more promptly and accurately than the conventional system. It should be noted that in the present specification and claims appended thereto, the term xe2x80x9cvalve-actuating systemxe2x80x9d is intended to encompass not only the valve-actuating mechanism and a drive source thereof but also intake valves and exhaust valves.
Preferably, the failure determination means determines that the valve-actuating system has failed, when the oxygen concentration is equal to or lower than a predetermined value.
When the failure-determining operation mode is carried out, if the valve-actuating system of the intake and exhaust valves associated with the part of the cylinders, which are caused to resume their operations, is normally operating, the concentration of oxygen in exhaust gases changes in an increasing direction. Therefore, according to this preferred embodiment, it is possible to properly determine whether or not the valve-actuating system has failed, by comparing a detected oxygen concentration with the predetermined value.
Preferably, the failure determination means determines that the valve-actuating system has failed, when an amount of change in the oxygen concentration in an increasing direction is smaller than a predetermined amount of change.
Under conditions in which the concentration of oxygen in exhaust gases is liable to be high, e.g. when the engine temperature is low, the concentration of oxygen in exhaust gases before execution of the failure-determining operation mode tends to have been drifted to a relatively high value. Even under such a condition liable to the drift, according to the preferred embodiment, the determination as to whether or not the valve-actuating systems have failed is carried out based on the amount of change in the oxygen concentration in the increasing direction, and hence it is possible to prevent erroneous determination of the failure of the valve-actuating system, whereby the accuracy of the failure determination can be further enhanced.
To attain the above object, according to a second aspect of the invention, there is provided a failure determination system for a variable-cylinder internal combustion engine including a plurality of cylinders each having at least one intake valve and at least one exhaust valve, and a valve-actuating system including a valve-actuating mechanism for actuating the at least one intake valve and the at least one exhaust valve of each of the cylinders, the engine being capable of switching a cylinder operation mode between an all-cylinder operation mode in which all of the plurality of cylinders are operated, and a partial-cylinder operation mode in which fuel supply to part of the plurality of cylinders is stopped and at the same time the part of the plurality of cylinders are made idle by stopping the valve-actuating mechanism from actuating the at least one intake valve and the at least one exhaust valve of each of the part of the cylinders.
The failure determination system according to the second aspect of the invention is characterized by comprising:
an oxygen concentration-detecting module for detecting a concentration of oxygen in exhaust gases exhausted from the plurality of cylinders;
a failure-determining operation mode-executing module for executing a failure-determining operation mode for causing the valve-actuating mechanism to resume actuation of the at least one intake valve and the at least one exhaust valve of at least one of the part of the cylinders in a state in which the fuel supply to the part of the cylinders is stopped, when the cylinder operation mode is switched from the partial-cylinder operation mode to the all-cylinder operation mode; and
a failure determination module for determining whether or not the valve-actuating system including the valve-actuating mechanism has failed, based on an oxygen concentration detected by the oxygen concentration-detecting module when the failure-determining operation mode is executed.
According to the second aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
Preferably, the failure determination module determines that the valve-actuating system has failed, when the oxygen concentration is equal to or lower than a predetermined value.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
Preferably, the failure determination module determines that the valve-actuating system has failed, when an amount of change in the oxygen concentration in an increasing direction is smaller than a predetermined amount of change.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
To attain the above object, according to a third aspect of the invention, there is provided a method of determining failure of a valve-actuating system of a variable-cylinder internal combustion engine including a plurality of cylinders each having at least one intake valve and at least one exhaust valve, and the valve-actuating system including a valve-actuating mechanism for actuating the at least one intake valve and the at least one exhaust valve of each of the cylinders, the engine being capable of switching a cylinder operation mode between an all-cylinder operation mode in which all of the plurality of cylinders are operated, and a partial-cylinder operation mode in which fuel supply to part of the plurality of cylinders is stopped and at the same time the part of the plurality of cylinders are made idle by stopping the valve-actuating mechanism from actuating the at least one intake valve and the at least one exhaust valve of each of the part of the cylinders.
The method according to the third aspect of the invention is characterized by comprising the steps of:
detecting a concentration of oxygen in exhaust gases exhausted from the plurality of cylinders;
executing a failure-determining operation mode for causing the valve-actuating mechanism to resume actuation of the at least one intake valve and the at least one exhaust valve of at least one of the part of the cylinders in a state in which the fuel supply to the part of the cylinders is stopped, when the cylinder operation mode is switched from the partial-cylinder operation mode to the all-cylinder operation mode; and
determining whether or not the valve-actuating system including the valve-actuating mechanism has failed, based on an oxygen concentration detected when the failure-determining operation mode is executed.
According to the third aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
Preferably, the determining step includes determining that the valve-actuating system has failed, when the oxygen concentration is equal to or lower than a predetermined value.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
Preferably, the determining step includes determining that the valve-actuating system has failed, when an amount of change in the oxygen concentration in an increasing direction is smaller than a predetermined amount of change.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
To attain the above object, according to a fourth aspect of the invention, there is provided an engine control unit including a control program for causing a computer to carry out determination of failure of a valve-actuating system of a variable-cylinder internal combustion engine including a plurality of cylinders each having at least one intake valve and at least one exhaust valve, and the valve-actuating system including a valve-actuating mechanism for actuating the at least one intake valve and the at least one exhaust valve of each of the cylinders, the engine being capable of switching a cylinder operation mode between an all-cylinder operation mode in which all of the plurality of cylinders are operated, and a partial-cylinder operation mode in which fuel supply to part of the plurality of cylinders is stopped and at the same time the part of the plurality of cylinders are made idle by stopping the valve-actuating mechanism from actuating the at least one intake valve and the at least one exhaust valve of each of the part of the cylinders.
The engine control unit according to the fourth aspect of the invention is characterized in that the control program causes the computer to detect a concentration of oxygen in exhaust gases exhausted from the plurality of cylinders, execute a failure-determining operation mode for causing the valve-actuating mechanism to resume actuation of the at least one intake valve and the at least one exhaust valve of at least one of the part of the cylinders in a state in which the fuel supply to the part of the cylinders is stopped, when the cylinder operation mode is switched from the partial-cylinder operation mode to the all-cylinder operation mode, and determine whether or not the valve-actuating system including the valve-actuating mechanism has failed, based on an oxygen concentration detected when the failure-determining operation mode is executed.
According to the fourth aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
Preferably, the control program causes the computer to determine that the valve-actuating system has failed, when the oxygen concentration is equal to or lower than a predetermined value.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
Preferably, the control program causes the computer to determine that the valve-actuating system has failed, when an amount of change in the oxygen concentration in an increasing direction is smaller than a predetermined amount of change.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.